1. Field of the Invention
The present invention relates to a method for modulating a three-phase pulse width in a motor control system, and more particularly to a three-phase pulse width modulation method and system that reduces a voltage generated between a three-phase alternating current AC motor and a ground, that is a common mode voltage.
2. Description of the Conventional Art
FIG. 1 is a diagram illustrating a three-phase pulse width modulation system for an AC motor in the conventional art.
As shown therein, a converter 1 converts a three-phase voltage to a direct current DC voltage and an inverter 2 receives the DC voltage through a couple of capacitors C1, C2 and modulates the received DC voltage to a three-phase pulse width system, the converter 1 and the inverter 2 consisting of six switching transistors T11-T16, T21-T26, respectively.
A control unit 4 outputs first pole voltage commands S.sub.U,S.sub.V,S.sub.W for switching the switching transistors T11-T16 of the converter 1 and outputs second pole voltage commands S.sub.A,S.sub.B,S.sub.C for switching the switching transistors T21-T26 of the inverter 2. A first driving unit 5 supplies driving signals D1-D6 to the switching transistors T11-T16, respectively, in accordance with the first pole voltage commands S.sub.U,S.sub.V,S.sub.W, and a second driving unit 6 applies driving signals D7-D12 to the switching transistors T21-T26, respectively, in accordance with the second pole voltage commands S.sub.A,S.sub.B,S.sub.C. For example, when the first pole voltage command S.sub.U is 1, the switching transistor T11 of the converter 1 is turned on and T14 is turned off, while if the second pole voltage command S.sub.A is 1, the switching transistor T21 of the inverter 2 is turned on and T24 is turned off.
Since the converter 1 and the inverter 2 are switched in accordance with the pole voltage commands of the control unit 4, a pattern of the three-phase pulse width modulation is determined by the pole voltage commands. Thus, how the pole voltage commands are outputted controls the three-phase pulse width modulation.
While, a voltage difference Vsg between a neutral point S of stator coils of an AC motor 3 and a ground is a common-mode voltage. Such a common-mode voltage causes many problems, which are mainly a leakage current flowing through a parasitic capacitor existing between the motor coils and a motor case, emission or conductive electrical interference, a shaft voltage and a bearing current, and dielectric breakdown of the motor due to an overvoltage. Damage caused by those problems has occurred in succession in the whole industrial world, and accordingly studies for reducing the common-mode voltage have been lively made.
With reference to FIGS. 2 and 3, there will be described the switching of the converter 1 and the inverter 2 in accordance with the pole voltage commands supplied from the control unit 4 and the relation between the switching of the converter 1 and the inverter 2 and the common-mode voltage.
Specifically, FIG. 2 is a vector diagram illustrating an output voltage vector of the converter and an output voltage vector of the inverter in the conventional art.
As shown therein, vectors V0-V7 are output voltage vectors of the converter or the inverter, each being determined by a value of the pole voltage command. A first digit of 100 in a vector V1 (100) indicates the pole voltage command S.sub.U or S.sub.A, a second digit thereof indicates the pole voltage command S.sub.V or S.sub.B, and a third digit thereof indicates the pole voltage command S.sub.W or S.sub.C. In other words, the first digit, which is 1 of 100, means that the upper switching transistor T11 coupled with the phase U of the converter 1 is turned on and the lower switching transistor T14 is turned off, or the upper switching transistor T21 coupled with the phase A of the inverter 2 is turned on and the lower switching transistor T24 is turned off. Thus, the converter and the inverter have eight output voltage vectors, respectively, in accordance with logic levels of the pole voltage commands, wherein vectors V1-V6 which are effective voltage vectors control the motor 3 and the other two vectors V0,V7 which are zero voltage vectors indicate that no three-phase voltage is supplied to the motor 3, whereby the motor 3 is not affected thereby.
FIG. 3 is a wave-form diagram illustrating an output of each unit in FIG. 1 when performing a three-phase symmetrical space-vector pulse width modulation.
As shown therein, on-intervals T.sub.U,T.sub.V,T.sub.W of the first pole voltage commands S.sub.U,S.sub.V,S.sub.W indicate switching intervals of the phases U, V, W, respectively, of the converter 1 and on-intervals T.sub.A,T.sub.B,T.sub.C of the second pole voltage commands S.sub.A,S.sub.B,S.sub.C indicate switching intervals of the phases A, B, C, respectively, of the inverter 2. In addition, Vc and Vi indicate the output voltage vectors of the converter 1 and the inverter 2, respectively, and a portion with oblique lines indicates the effective voltage vector. Vsg, as described above, is the common-mode voltage. The effective voltage of the output voltage vector Vc of the converter 1 exists in an interval T.sub.U -T.sub.W until the pole voltage command S.sub.W becomes turned on after the pole voltage command S.sub.U is turned on, while the effective voltage of the output voltage vector Vi of the inverter 2 exists in an interval TA-TC until the pole voltage command S.sub.C becomes turned on after the pole voltage command S.sub.A is turned on. The common-mode voltage Vsg is determined by the switching of the converter 1 and the inverter 2, the common-mode voltage Vsg, which exists during the interval in which the output voltage vector Vc of the converter 1 is the effective voltage, having six different changes during a control cycle Tz. The size of the common-mode voltage Vsg is 1/3 of a DC bus voltage Vdc which is a voltage between a connection point of condensers C1, C2 and a connection point of the two switching transistors of the inverter 2.
As described above, among the problems caused by the common-mode voltage, particularly many problems occur due to the pulse in accordance with the change of the common-mode voltage dVsq/dt, the pulse being generated at a point of time when the common-mode voltage Vsg varies.
To solve the above problems due to the common-mode voltage, there has been introduced a method that offsets the common-mode voltage by using a filter and a common-mode choke/transformer or additionally providing a leg to the three-phase pulse width modulation system of the conventional AC motor. However, to perform the above conventional method, a separate hardware must be additionally supplied, which results in increase in the weight and volume of the system and the manufacturing cost thereof, and the complication of the control, thereby leading to the deterioration of the system reliability. In addition, since the filter and common-mode transformer must be suitably designed to each application field, it is difficult to apply to the industrial circles.